While conventional brush-commutated DC motors may have numerous advantageous characteristics such as convenience of changing operational speeds and direction of rotation, it is believed that there may be disadvantages such as brush wear, electrical noise or RF interference caused by sparking between the brushes and the segmented commutator, that may limit the applicability of such brush-commutated DC motors in some fields such as the domestic appliance field. Electronically commutated motors, such as brushless DC motors and permanent magnet motors with electronic commutation, have now been developed and generally are believed to have the above discussed advantageous characteristics of the brush-commutated DC motors without many of the disadvantages thereof while also having other important advantages. Such electronically commutated motors are disclosed in the David M. Erdman U.S. Pat. Nos. 4,005,347 and 4,169,990 and Floyd H. Wright U.S. Pat. No. 4,162,435. These electronically commutated motors may be advantageously employed in many different fields or motor applications among which are domestic appliances, e.g., automatic washing or laundry machines such as disclosed in the aforementioned co-pending U.S. patent applications, and in Ser. No. 077,784 filed Sept. 21, 1979 (now U.S. Pat. No. 4,327,302) and Ser. No. 141,268 filed Apr. 17, 1980 (now U.S. Pat. No. 4,390,826).
Laundry machines as there disclosed are believed to have many significant advantages over the prior art laundry machines which employ various types of transmissions and mechanisms to convert rotary motion into oscillatory motion to selectively actuate the machine in its agitation or washing mode and in its spin extraction mode, and such prior art laundry machines are believed to be more costly and/or complicated to manufacture, consume more energy, and require more servicing. Laundry machines with electronically commutated motors require no mechanical means, other than mere speed reducing means, to effect oscillatory action of the agitator, and in some applications the spin basket might be directly driven by such a motor. While the past control systems, such as those disclosed in the aforementioned coassigned applications for instance, undoubtedly illustrated many salient features, it is believed that the control systems for electronically commutated motors in general, and for such motors utilized in laundry machines, could be improved. In some of the past control systems, the position of the rotatable assembly (i.e., the rotor) of the electronically commutated motor was located by sensing the back emf of one of the winding stages on the stationary assembly (i.e, the stator) thereof with reference to a neutral conductor voltage of the motor. In some of the past electronically commutated motors, however, a neutral conductor may not have been readily available, so it is believed that a control system for these motors would be desirable. Some of the past control systems may have also included provision for starting an electronically commutated motor by supplying a rotating field to the rotor thereof by means of a voltage offset input to such control system. This offset was provided by an analog input, and it is believed to have introduced an offset error into the past control systems. Some of these past control systems also may have used an integrator to determine the angular position of the rotatable assembly of the past electronically commutated motors, which integrator was reset by an analog circuit after each commutation of such motors. Resetting the integrator using analog signals is believed to have required the use of components which may not advantageously lend themselves to subsequent application of integrated circuit technology to the entire control system. The above mentioned past control systems also may have lacked electronic circuit breaker and voltage regulation capabilities which are believed to be desirable features for a control system.
Some of the past control systems for electronically commutated motors, such as some of those shown in the patents and applications mentioned above for instance, used a pair of driving transistors, called upper and lower transistors, for each winding stage of such motors. In these past control systems, the upper transistors or the lower transistors, but not both, were pulse width modulated to control the electronically commutated motor speed. However, this is believed to have resulted in uneven usage of the drive transistors and affected the position sensing of such past control systems. It is believed that pulse width modulating the drive transistor associated with the winding stage of the electronically commutated motor which remains on after commutation, as opposed to the winding stage which was just turned on at the commutation, results in faster transfer of the winding current to the winding stage which was just commutated on and in less electrical noise in the back emf signal of the third winding stage after zero crossing of said back emf signal. It is also believed that this faster transfer of winding current is preferable in applications where motor current is being controlled or where the motor has a high inductance. However, it is further believed that previous control systems did not permit this faster transfer of winding current after each commutation.
In using electronically commutated motors in the laundering machine application, for example, it may be desirable to reverse the direction of rotation of the motor from time to time, such as when the machine is in the agitate mode, rather than use a heavy and expensive transmission. However, with some of the aforementioned past motor control systems, the act of reversing the motor is believed to have resulted in a momentary current surge through the drive transistors which may have undesirably heated the drive transistors. Moreover, it is desirable in the laundering machine application, among others, to operate the electronically commutated motor at two different speeds. However, it is believed that this may have led in some of the prior control systems to inaccuracies in the sensing of rotor position. For example, it is believed that a control system which integrated the back emf signal to obtain a rotor position signal might give different position results for low speed and high speed operation for the same actual rotor position.